The present invention relates to an improved heater clamp arrangement for use in an injection molding machine, especially in an injection molding machine of the hot runner type, particularly useful in securing a heater element in a desired position around a nozzle body for feeding liquified plastic to an injection gate of a mold cavity.
To maintain the fluidity of the resident plastic material during closure of the injection gate, enabling its use in a following molding cycle, it is desirable to form a terminal part of the sprue channel as an axial bore of a nozzle of good thermal conductivity closely surrounded by an electrical heating element. Some nozzle manufacturers go through costly processes to integrate heaters into the nozzle body. The primary disadvantage with this approach however is that should the heater get damaged the nozzle is lost as well. Thus, it is preferred that the heating element be removably mounted onto the nozzle body for ready replacement. One of the primary challenges associated with this technique is holding the heating element firmly against the external surface of the hot runner nozzle housing so as to maximize heater performance.
To obtain effective and efficient heat transfer from the heater to the heated part, it is necessary to have close contact between them. This is generally accomplished by use of a clamping device such as a metal shroud which encompasses the heater body and is screwed together at its ends to draw the heater tighter against the nozzle to be heated. This method has an advantage of being simple, using common tools such as a screw driver to tighten or loosen the heater. However, in cases where the heater must be installed in a confined area it can be difficult to access the screw head for tightening purposes once it is in its correct orientation in the molding machine. Typically, the obstruction is the mold plate adjacent to and surrounding the heater. In such cases, additional clearance is often machined into the plate to permit the tightening tool to reach the screw head.
In situations where it is prohibitive to add clearance machining because it will compromise the strength of the mold plate or reduce the plate material available to back-up and support other mold components, a design as shown in U.S. Pat. No. 4,968,247, issued Nov. 6, 1990, permits tightening of the heater by way of a cam actuated clamp housing. This design permits a tool to approach from the axial direction of the heater, thus requiring no special clearance for the tightening tool. While this is an improvement for ease of assembly and structural integrity of the mold plate, there is still the need to cut a small pocket of clearance for the cam mechanism which stands outside of the cylindrical profile of the outer surface of the heater. Likewise there is a variety of other tightening devices available for use on the market, which also invariably add substantially to the outer diameter of the heater.
Some coil heaters on the market have no external clamping mechanism but use the spring nature of the coil to hold its position on the nozzle. By design, the element must be heavy (thick) and stiff, to maintain its size, shape and gripping ability. Thus, it can be difficult to install or remove in a confined space or without special tools.
Clampless heaters show no clamp mechanism at all, but instead rely on an extremely precise fit between the heater and the nozzle, thus requiring no extra clearance other than for the outer diameter of the heater. Aside from the high cost of manufacturing both fitting diameters, there are additional drawbacks, for example, it can become necessary to engineer additional devices to trap the heater on the nozzle to prevent it from slipping axially away from its installed position during handling or movement of the mold. These heaters also tend to have a thick wall section, in the order of 4 mm for heaters with a 12-42 mm internal diameter. Also, should any burrs or surface imperfections exist on the mating surfaces the heater can seize on the nozzle and become very difficult to remove without damage to the heater or the nozzle.
It would be highly desirable to provide a simple, cost effective way to clamp a heater to an injection molding nozzle or other channel means for transferring molten plastic while increasing the outer diameter of the channel means as little as possible. Accordingly, this is the principal objective of the present invention.
A further object of the present invention is to provide an improved heater clamp arrangement as aforesaid which is easy and convenient to use in a confined area with little space and which is operative to clamp the heater to the channel means in the hot or cold condition.
Further objects and advantages of the present invention will appear hereinbelow.